Hydrogen odorants and odorant selection method

ABSTRACT

The present invention provides a method for evaluating the properties of hydrogen to improve the safety of hydrogen fuel, and provides a method for selecting proper odorants for hydrogen. Odorized hydrogen containing suitable odorants in appropriate concentrations with hydrogen are also provided.

This is a divisional of prior application Ser. No. 10/637,608, filed onAug. 11, 2003 the entire disclosures of which are incorporated byreference herein.

This application claims priority from U.S. Provisional Application60/402,664 entitled “Hydrogen Odorization System and Method” filed onAug. 13, 2002. The entire contents of which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method for evaluating theproperties of hydrogen to improve the safety of hydrogen fuel, and moreparticularly to a method of selecting a proper odorant for hydrogen.

2. Description of the Prior Art

Hydrogen is considered by many to be the fuel of the future due to itshigh availability, very high calorific value, wide flammability limitsand non-polluting nature. However, the highly combustible nature ofhydrogen poses a great hazard creating a number of problems with itssafety and handling. Therefore, leak detection in hydrogen systems iscritical for any hydrogen application. Pure hydrogen is a colorless andodorless gas. There are many different types of mechanical ‘hydrogen gasdetectors’, but as with any mechanical device, these detectors are alsoprone to mechanical failure. Even the most durable hydrogen sensors aretoo costly and cumbersome for automotive use. Existing sensors are tooeasily jostled, and their reactive metals, which include expensivenoble-metals such as palladium, are ruined by contact with gases andparticles that are common on the road. Considering the dangersassociated with an extremely flammable gas like hydrogen, it becomesnecessary to have an odorant for this fuel just as there are mercaptansfor detecting natural gas leaks, accepted widely as a means ofmaintaining safety. The present invention alleviates safety concerns byodorizing hydrogen.

While gasoline, diesel, gasoline hybrid, and diesel hybrid vehicles areprevalent, and, electric and fuel cell vehicles are becoming more commonin the transportation industry, vehicles do not currently incorporatehydrogen odorants or odorant removal systems. Some of the shortcomingsand disadvantages associated with gasoline and diesel vehicles includepollution and dependency upon imported oil. Current electric vehicles,unfortunately, require a long battery charge time when not in operation.The main barrier to a widespread adoption of fuel cell vehicles is alack of re-fueling infrastructure.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a model toprovide an appropriate odorant for hydrogen.

It is a further object to provide criteria for selecting a properodorant to minimize catalyst poisoning.

Finally, it is an object of the invention to provide a method ofintegrating the odorized hydrogen into an existing fuel cell system.

According to a first broad aspect of the present invention, there isprovided a hydrogen composition comprising: hydrogen gas; and anodorant, said odorant having a vapor pressure greater than 0.5 psi atstandard conditions, having a smell detectable at less than 1 ppm by ahuman nose, and being in a vapor phase at detectable concentration underhydrogen storage conditions at pressures of 6000 psi.

According to a second broad aspect of the present invention, there isprovided a method for detecting a hydrogen gas leak from a containercomprising; providing a container containing a hydrogen composition; anddetecting a leak from said container when the smell of an odorantpresent in said hydrogen composition is sensed, wherein said hydrogencomposition comprises hydrogen and said odorant, said odorant having avapor pressure greater than 0.5 psi, having a smell detectable at lessthan 1 ppm by a human nose, and being in a vapor phase at detectableconcentrations under hydrogen storage conditions at pressures of 6000psi.

According to a third broad aspect of the present invention, there isprovided a method of making a hydrogen composition comprising: providinghydrogen gas; and mixing an odorant with said hydrogen gas to form saidhydrogen composition, said odorant having a vapor pressure greater than0.5 psi, having a smell detectable at less than 1 ppm by a human nose,and being in a vapor phase at detectable concentration at a pressure of6000 psi.

Other objects and features of the present invention will be apparentfrom the following detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic demonstrating the principle of diffusivetransport;

FIG. 2 is a graph illustrating olfactory power of n-alcohols;

FIG. 3 is a chart of the relative detectability of Group 16 cogeners;

FIG. 4 is a chart of the vapor pressure-olfactory power relationshipgeneralized for all odorants;

FIG. 5 is a model examining a hydrogen leak in a confined space inaccordance with a preferred embodiment of the invention;

FIG. 6 is a second model examining a hydrogen leak in a confined spacein accordance with the preferred embodiment of the invention;

FIG. 7 is a chart of the hydrogen concentration profile as a function oftime, as hydrogen approaches the lower flammability limit forobservation points A and D (Observation points A and D are referencedfrom FIGS. 5 and 6);

FIG. 8 is a chart of the hydrogen concentration profile as a function oftime, as hydrogen approaches the lower flammability limit forobservation points B and E (Observation points B and E are referencedfrom FIGS. 5 and 6);

FIG. 9 is a chart of the hydrogen concentration profile as a function oftime, as hydrogen approaches the lower flammability limit forobservation points C and F (Observation points C and F are referencedfrom FIGS. 5 and 6);

FIG. 10 is a schematic of a system design with odorant adsorbersarranged prior to a solid storage unit;

FIG. 11 is a schematic of a system design with odorant adsorbersarranged prior to a fuel cell stack;

FIG. 12 illustrates a fixed bed flow system;

FIG. 13 shows the effect of varying mass transfer coefficients onbreakthrough curve shape;

FIG. 14 illustrates the effect of varying effective diffusioncoefficients on breakthrough curve shape;

FIG. 15 shows typical adsorption breakthrough curves as predicted by theRosen equation showing the effect of increasing the Henry's Law constant(equilibrium adsorption capacity of adsorbent);

FIG. 16 shows the breakthrough curves based on H₂S adsorption over highsurface area; and

FIG. 17 is an odorant selection matrix.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

DEFINITIONS

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provided below,unless specifically indicated.

For the purposes of the present invention, the term “adsorption” refersto adhesion of molecules of a gas, liquid or dissolved substance to asurface.

For the purposes of the present invention, the term “breakthrough time”refers to the time required for the odorant adsorber to reach itscapacity wherein the odorant is detected in the hydrogen effluent at theadsorber outlet.

For the purposes of the present invention the term “container” refers toany type of container that may contain hydrogen or through whichhydrogen may pass, including for example a metal container, a fuel cell,a pipe, etc.

For the purposes of the present invention, the term “convection zone”refers to the volumetric space by which the leak velocity controlshydrogen transport or hydrogen composition transport.

For the purposes of the present invention, the term “detecting device”refers to any type of detecting device capable of detecting the presenceof the odorants of the present invention at the concentrations at whichthe odorants are present in the hydrogen compositions of the presentinvention or are present in leaks from containers containing thehydrogen compositions of the present invention.

For the purposes of the present invention, the term “fixed bedadsorption” refers to a process whereby odorant is removed from odorizedhydrogen by flowing odorized hydrogen through a stationary or “fixed”adsorbent bed.

For the purposes of the present invention, the term “flammabilitylimits” refers to a discrete range of fuel/air mixtures whereby a flamewill propagate only within this defined range. The term “lowerflammability limit” refers to the leanest fuel/air mixture that willallow steady flame propagation. The term “upper flammability limit”refers to the richest fuel/air mixture that will allow steady flamepropagation.

For the purposes of the present invention, the term “fluid” refers toany gas or liquid.

For the purposes of the present invention, the term “fuel cell” refersto a device that converts chemical energy directly into electricalenergy.

For the purposes of the present invention, the term “hydrogenconcentration front” refers to the leading edge of a hydrogen cloud asit diffuses through another fluid.

For the purposes of the present invention, the term “hydrogenreflection” refers to the reflection of hydrogen gas off of a barrier.

For the purposes of the present invention, the term “leak zone” refersto the path created by the flow of hydrogen leak through a hole.

For the purposes of the present invention, the term “observation point”refers to points within a model selected to compare and study thediffusion of hydrogen through a room or other confined space.

For the purposes of the present invention, the term “odorant” refers toa chemical compound with a smell detectable by a human nose.

For the purposes of the present invention, the term “odorantconcentration front” refers to the leading edge of an odorant cloud asit diffuses through another fluid.

For the purposes of the present invention, the term “odorant loading”refers to the concentration of odorant added to hydrogen.

For the purposes of the present invention, the term “source point”refers to a point representing a hydrogen leak in a room.

For the purposes of the present invention, an odorant is considered “notharmful to humans” if exposure to an odorant is within permissibleexposure limits, or within threshold limit values, or within workplaceenvironmental exposure guidelines as set by regulatory agencies such asOSHA, NIOSH, and AIHA respectively.

DESCRIPTION

The United States safely uses about 90 billion cubic meters (3.2trillion cubic feet) of hydrogen every year primarily for refiningpetroleum and for manufacturing industrial commodities, such as ammonia.In addition, chemical, metallurgical, semiconductor, fats and oils, foodprocessing, glass, and electronic industries utilize smaller amounts ofhydrogen. Comparatively little hydrogen is currently being used as afuel or as an energy carrier. An energy infrastructure that useshydrogen as an energy carrier—a concept called the “hydrogen economy”—isconsidered the most likely path toward a full commercial application ofhydrogen energy technologies. The transition to a hydrogen economy isdriven by the need to reduce pollution from the transportation sectorthat uses fossil fuels as a primary energy source. The production,distribution and utilization of fossil fuels significantly contributesto pollution problems in the U.S. and around the world.

The hydrogen fuel cell is the most promising future power source due toits high-energy efficiency and zero emission potential. Hydrogen'spotential use in fuel and energy applications includes poweringvehicles, running turbines or fuel cells to produce electricity, andgenerating heat and electricity for buildings. The use of hydrogen as afuel and energy carrier will require an infrastructure for safe andcost-effective hydrogen transport and storage. Some automobilemanufacturers have developed hydrogen-powered vehicles, however themarket for these vehicles is limited by the lack of hydrogeninfrastructure. Commercial application of hydrogen fuel cells in theautomotive industry will be an important driver of the hydrogen economy.

Currently, NASA's space program is a primary example of hydrogen's useas a fuel. Liquid hydrogen has propelled the space shuttle and otherrockets since the 1970's. The Challenger space shuttle accident has beena reminder of the safety risks involved when hydrogen is used as fuel.Aside from classical disasters such as the Challenger accident, hydrogenhas an excellent safety record and is as safe for transport, storage anduse as many other fuels. Nevertheless, safety remains a top priority inall aspects of hydrogen energy. The Department of Energy's HydrogenEnergy program delivers an expectation that hydrogen will become a majorenergy source derived from renewable resources. The present inventionintegrates hydrogen odorization and onboard vehicle odorant adsorptioninto a fuel cell system. While this invention is presented forconsideration in transportation, it is understood that the design mayalso be modified for stationary fuel cell systems. This design is uniqueas it develops hydrogen odorization as a safety consideration.

At fueling stations today, hydrogen dispensers are capable oftransferring hydrogen to fuel cell vehicles at pressures up to 6,000 psiat ambient temperature. Hydrogen storage and dispensing infrastructureis a key safety concern due to the dangers of a high-pressure hydrogenleak combined with hydrogen's explosive nature at and around a fuelingstation. Having odorized hydrogen available at fueling stations willincrease the safety of handling hydrogen. In terms of on-board vehiclestorage, a majority of fuel cell vehicles in operation today utilizecompressed gas storage tanks. Enhancing the safety of compressedhydrogen cylinders is also a critical need. Odorized hydrogen wouldenhance the safety of compressed hydrogen cylinders, given a slow orpinhole leak scenario. Other technologies that store hydrogen on-boardin solid-state are under investigation (metal hydrides, carbonnanotubes, and glass microspheres) and are inherently safer thancompressed or liquid storage of hydrogen. However, with any type ofon-board storage device the issue remains with maintaining safety athydrogen filling stations. Odorized hydrogen will undoubtedly improvesafety at filling stations.

Hydrogen fuel cells use platinum to electrochemically oxidize hydrogenat the anode of the cell. If the platinum's active sites are occupied bya molecule other than hydrogen, the activity of the catalyst, and hencethe efficiency and performance of the fuel cell, dramatically decreases.This is referred to as “poisoning of the catalyst”. Similarly, on-boardsolid-state storage devices, such as metal hydrides, carbon nanotubes,or glass microspheres may be “poisoned”. Any odorant chosen for hydrogenshould not poison the storage material, nor should the odorant inhibitfuel cell performance or poison the fuel cell catalyst. If the odorantis found to affect fuel cell catalyst performance, or performance of asolid-state storage device, the odorant should be removed from hydrogenbefore entering the device. In a hydrogen fuel cell application, theodorant may be removed by a fixed bed adsorption system.

Hydrogen odorization focuses on the development of an odorant and aviable method of odorization that is compatible with hydrogen fuelsystems without interfering with the components and energy conversionefficiency of the system. A suitable odorant needs to have a unique odorand must be non-toxic to both human beings and the environment at therequired concentration.

Hydrogen, like natural gas, faces similar industrial problems in itsapplication, such as storage, transmission and distribution. In order toprotect public welfare and safety, hydrogen needs to be odorized beforeits large-scale application/utilization by the general public. Naturalgas has a set of mature and effective odorization systems for itsapplication, governed by specific federal regulations; however, thereare currently no special regulations governing the odorization ofhydrogen. Current regulations regarding the safe use, transportation andstorage of natural gas are necessary to understand constraints andlimitations in choosing a suitable odorant for the hydrogen economy. Thecodes and standards developed to date have been implemented due tospecial safety concerns related to the storage, transmission anddistribution of natural gas. Mixtures of hydrogen and odorant,therefore, should also comply with current regulations for natural gasodorants. International standards drafted by the International StandardsOrganization for Hydrogen Technologies (ISO/TC 197) with respect togaseous hydrogen applications in the transportation sector will provideregulations for storage tanks, filling stations, vehicles and pumpconnectors, as well as hydrogen product specifications. These draftregulations help gain an understanding of compatibility issues that mayarise as a result of adding an odorant. The Department of Transportation(DOT) and Occupational Safety and Health Administration (OSHA) havejurisdiction over domestic hydrogen regulations.

To odorize flammable gas, compatible odorants must be carefullyselected. Compounds meeting specific criteria may be chosen withconsideration for economic factors such as technical feasibility andproduction cost. An odorant favorable for this purpose should have lowsolubility in water, good oxidative stability, a minimum defined odorthreshold, a minimum defined vapor pressure, and a minimum defineddiffusivity. Additionally toxicity, combustibility and other propertiesof combustion products should be considered. If a single componentcannot satisfy all the requirements, blends of two or more componentsmay be used.

Physical and chemical properties of hydrogen that are important to thedevelopment of a proper hydrogen odorant include:

Lower Flammability Limit (% by volume/air): 4.0 Lower Detonation Limit(% by volume/air): 18.3 Stoichiometric Mixture (% by volume/air): 29.6Upper Flammability Limit (% by volume/air): 75.0 Upper Detonation Limit(% by volume/air): 59.0 Minimum Ignition Energy (mJ): 0.017Auto-Ignition Temperature (° C.): 520° Density @ Standard T/P (Kg/m³):0.0827 Viscosity @ Standard T/P (10⁻⁶ Pa*s): 8.814 Diffusion Coefficientin Air (cm²/s): 0.76

Hydrogen has a very low density enabling it to readily disperse and mixwith air to create explosion or ignition hazards. A hydrogen molecule isvery small and has a high diffusion coefficient both in other gases andin solids. When considering hydrogen leaks, its high buoyancy generallyaffects gas motion considerably more than its high diffusivity. However,the diffusive and dispersive characteristics limit any explosion orignition hazards in a well-ventilated area. The low viscosity ofhydrogen, and its respective flow rate, enhances the hazard of leaksthrough porous materials, fittings, and seals (i.e. hydrogen has a flowrate approximately 25% higher than methane through the same leak withthe same associated pressure drop). Density, diffusivity, and viscositycharacteristics should be taken into consideration to select materialsused for hydrogen storage devices and to consider compatibility with asuitable odorant.

Characteristically, a higher diffusivity coefficient describes acompound with a lower molecular weight. A compound's diffusivity isdependent upon molecular weight and temperature, rather than theconcentration. However, the rate of molecular diffusive transport of acompound is determined from its diffusion coefficient and theconcentration gradient of the gaseous compound in an environment andthereby the process of molecular diffusion. As the concentration of acompound within an environment increases so does the frequency ofmolecular collision thus increasing the rate of mixing. Referring toFIG. 1, if point * is the source of gaseous release, then higherconcentration lies around that point * during a leak event than at anydistance away from that point *. Therefore, so long as the distance, x,increases, the concentration of molecules decreases and the magnitude ifmolecular diffusion (diffusive mixing) decreases. Brownian motiondescribes the behavior wherein the concentration gradient between anydistance, x, and reference point decreases, resulting in slower mixing(lower collision frequency). Hydrogen has its own diffusion coefficient(0.76 cm²/s) by which the magnitude of mixing decreases with distance ata constant rate governed by the change in concentration. Therefore, alonger time is necessary to reach a flammable concentration of H₂ at 4%as distance, x, increases away from the source.

The minimum ignition energy of hydrogen is very low. Since most ignitionsources generate more than 10 mJ of energy, most fuels would ignite ifthe fuel/air ratio reaches the lower flammability limit. Therefore, theminimum ignition energy, the lower flammability, and detonation limitsare important characteristics when selecting a suitable odorant.

Another important property of an odorant is its odor threshold. Asuitable odorant should be detectable by the human nose at very lowconcentrations. In fact, the odor should be detected as soon as hydrogenbegins to leak. In order for a compound to be considered as a suitableodorant, it should have a low odor threshold in the gas phase. Thisfactor is important because the odorant will be present as a gas alongwith hydrogen in the event of a leak. A critical issue in hydrogenodorization is phase compatibility between hydrogen and the odorant athigh pressure. To ensure simultaneous outflow in case of a leak, bothcomponents should be in the same phase and well blended. Some odorthreshold values may be found in the Handbook of Hazardous ChemicalProperties. In addition, the American Industrial Hygiene Association(AIHA) publishes a book of odor threshold values (Odor Thresholds forChemicals with Established Occupational Health Standards).

Physical properties, chemical properties, health hazard information, andodor threshold may be considered when developing the criterion basis forselecting hydrogen odorants. A vast myriad of organic compounds existthat may potentially be used as odorants for hydrogen fuel. The possiblecompounds have variable odor potency, odor threshold values, toxicity,and physical properties. Certain functional groups, such as themercaptans, have high potency odors. According to published standardizedhuman olfactory thresholds, mercaptans are among the strongest smellingcompounds known. The stench of mercaptans is a key reason why thesecompounds were chosen for natural gas odorization. The following is alist of functional groups that may be used to highlight key factors forolfactory thresholds in humans. Standard olfactory power refers to theminimum concentration at which a particular compound is detectable bythe average person. The mathematical definition is given as the negativelog of odorant concentration expressed in volumetric fraction: pOl=−log[odorant]. Based on this definition an olfactory power of “0/6/9”corresponds to “odorlessness/1 ppm/1 ppb” threshold levels,respectively. In air, a compound with a greater olfactory power may bedetected at lower concentration levels.

TABLE 1 Functionality and Odor Potency of Low Molecular Weight CompoundsFunctionality Compound Olfactory Power (pOl) hydrocarbons ethane 2.00propane 2.57 butane 3.69 halides chloromethane 4.99 ethylchloride 5.39alcohols methanol 3.85 ethanol 4.54 1-propanol 5.62 esters methylformate 4.03 methyl acetate 5.21 ketones acetone 4.84 aldehydesformaldehyde 6.06 acetaldehyde 6.73 amines methylamine 7.73dimethylamine 7.09 ethylamine 6.49 diethylamine 6.73 propylamine 7.96thiols methyl mercaptan 8.98 (mercaptans) ethyl mercaptan 8.97 isobutylmercaptan 8.95 t-butyl mercaptan 9.48 sulfides dimethylsulfide 8.65methylethylsulfide 8.42 diethylsulfide 8.41 selenides diethylselenide9.13 selenols ethylselenol 10.74From Table 1 and based on the definition of olfactory power, the abovesulfur compounds are detectable at levels of parts-per-billion (ppb). Interms of olfactory power, most other functional groups are not aspotent. The amines are also potent and detectable atsub-parts-per-million levels and notably, compounds containing seleniumare the most potent. Other functional groups considered include:tellurides, phosphines, boranes, nitroso compounds, nitro compounds,enamines, sulfoxides, sulfones and imines. An important trend extractedfrom this data set is that, in general, as molecular weight (and lipidsolubility) increases within one functionality (i.e. a homologousseries), olfactory power also increases. This is illustrated in FIG. 2,which shows the olfactory power of a homologous series of n-alcohols.Another important trend is extracted from FIG. 3, where the olfactorypower of Group 16 cogeners (oxygen vs. sulfur vs. selenium) are plottedfor ethanol, ethyl mercaptan, and ethyl selenol. It may be seen fromFIG. 3 that ethyl mercaptan has 30,000 times higher detectability ascompared to ethanol, and furthermore, ethyl selenol has 67 times higherdetectability as compared to ethyl mercaptan. Thus, the trend when goingfrom oxygen to sulfur to selenium (down a periodic Group), is anincrease in olfactory power. Ethyl selenol is therefore a better odorantcandidate as compared to sulfur or oxygen cogeners. The trend, wherebyolfactory power increases with molecular weight, is important whenselecting an appropriate odorant for hydrogen. The importance ofolfactory power is illustrated when considering issues such as odorantvapor-liquid equilibrium at high pressure and odorant dispersion anddiffusion characteristics relative to hydrogen. In terms of odorantvapor-liquid equilibrium, the odorant should have an appropriate vaporpressure, such the odorant remains in the vapor phase at detectableconcentrations under high pressures. Generally, smaller organiccompounds have higher vapor pressure. Both olfactory power and vaporpressure may be correlated to determine whether an odorant remains inthe vapor phase at detectable concentrations at high pressure. In termsof odorant dispersion and diffusion, the odorant should be capable offast diffusion in order to provide ample time for human sensorydetection, before hydrogen reaches explosive or flammability limits. Ingeneral, to meet diffusion and dispersion requirements, an odorantshould be a small compound. It becomes apparent that there is a need forsmall organic compounds with high olfactory power to meet odorantvapor-liquid equilibrium and odorant dispersion and diffusionrequirements of the system. Despite the aforementioned needs for smallodorant compounds, large organic compounds generally have a higherolfactory power. Therefore, the odorant selection criteria shouldemphasize selection of an odorant based on its small molecular size andhigh olfactory power.

A useful class of compounds for the odorization of hydrogen isorganoselenium compounds. Like their organosulfur cogeners,organoselenium compounds pack significant stench power. In most cases,the olfactory power of organoselenium compounds even exceeds that ofstructurally similar organosulfur compounds. The stench oforganoselenium compounds has been described as even more putrid andpenetrating as compared to organosulfur compounds. Organoseleniumcompounds may be reactive under oxidative conditions. Selenols tend toreact with each other in the presence of oxygen to form diselenidesgiven in the following formula:2R—SeH+O₂→R—Se—Se—R+H₂O

Although auto-oxidation may occur, the reaction is slow at ambientconditions. Auto-oxidation should not be a major concern since theresultant diselenide retains a potent stench.

Other factors to consider with organoselenium compounds are cost andavailability, as well as environmental and health hazards. Selenium isan essential element for normal human biological development and isfound in elemental form in daily vitamin supplements. Selenium is alsoan important component of antioxidant enzymes that protect cells againstthe effects of free radicals that are produced during normal oxygenmetabolism. Furthermore, selenium is essential for normal functioning ofthe immune system and thyroid gland. At larger doses, however, seleniumis known to cause a medical condition called selenosis. Selenosis occurswhen selenium is ingested in quantities of 5 mg/day for the averageweight person. If a catastrophic leak occurred from a hydrogen storagetank containing 10 kg of hydrogen loaded with 30 ppm ethylselenol, thetotal release of selenium would be 0.22 g of selenium. This amount ofselenium would not be expected to cause a health hazard. The cost ofselenium is $0.65/g and is available in commercial quantities.Therefore, a selenol odorant loaded at 30 ppm into 10 kg hydrogen wouldadd $0.14 to the cost of the hydrogen.

As stated above, it is crucial to understand the compatibility ofhydrogen and an odorant. Therefore, examination of how an odorant reactsto hydrogen's storage conditions is necessary. At low concentrations anodorant must remain in the vapor phase at detectable concentrationsunder high-pressure hydrogen storage conditions. To quantitativelyvalidate the concentration of an odorant in the gas phase at highpressure, first order phase equilibrium equations may be used.

When setting up the system, it is assumed that only two componentsexist, hydrogen (H₂) and an odorant (A). It is also assumed that theodorant is saturated. This means that at the constant pressure whenodorant is added, the amount of odorant in the vapor phase remainsconstant. The equation used to find the fraction of the vapor phasetaken solely by the odorant is Raoult's Law:x_(A)P_(A) ^(sat)=y_(A)P

Where, x_(A) is the odorant's fraction of the liquid phase, y_(A) is theodorant's fraction of the vapor phase, P_(A) ^(sat) is the vaporpressure of the saturated odorant, and P is the total pressure withinthe system. It takes extreme pressures on the order of 150,000 psi tocondense hydrogen at room temperature. From this knowledge it is assumedthat no hydrogen will exist in the liquid phase, therefore x_(A) willequal one:x_(A)=1And the equation for y_(A) becomes:

$y_{A} = \frac{P_{A}^{sat}}{P}$

With these values, the concentration of odorant may be found at ambientconditions for the storage conditions of 6,000 psi. To calculate thepressure needed to produce a certain saturated concentration of odorantin the vapor phase:

$P = \frac{P_{A}^{sat}}{y_{A}}$${Where},{y_{A} = \frac{\left\lbrack {{odorant}\mspace{14mu}({ppm})} \right\rbrack}{1 \times 10^{6}}}$The phase calculations for possible odorants are listed below in Table2.

TABLE 2 Odorant Vapor Saturated Odorant Pressure P_(A) ^(SAT)Concentration y_(A) Odorant (psi @ 25° C.) (ppm @ 6000 psi, 25° C.)pyridine 0.4 67 t-butyl mercaptan 3.5 583 ethyl mercaptan 10.2 1705dimethyl selenide 4.6 773 ethyl selenol* ~5 ~833 propyl amine 6.0 999dimethyl amine 29.4 4892 trimethyl amine 31.0 5172 *estimated valuebased on known vapor pressures of structurally similar compounds

T-butyl mercaptan and ethyl mercaptan are used for comparison sincethese compounds are common odorants for natural gas. From this analysis,it is noted that odorants such as t-butyl mercaptan are present in thevapor phase at 6000 psi up to a maximum concentration of approximately580 ppm. For compounds with low vapor pressures, there becomes a limitto the amount of odorant present in the gas phase at high pressure. Forexample, pyridine will be present in the gas phase at 6000 psi at amaximum concentration of only 67 ppm. It should be recognized that athigh pressures, vapor-liquid equilibrium deviate from linearity andtherefore, this first order equation should be used conservatively. Forthe odorant criteria, conservatively stated, a suitable odorant may havea vapor pressure greater than 0.5 psi.

Olfactory power provides an additional basis on which to narrow thepossibilities of potential odorants. Based on saturated odorant vaporphase concentration at high pressure, there is a minimum olfactory powerrequired of an odorant. Olfactory power first determines odorant loadingin the fuel, and odorant loading is further affected by phaseequilibrium during fuel storage at high pressure. In regard to phaseequilibrium, the odorant may remain in the vapor phase at 6000 psi onlyat equilibrium limited vapor saturation. Phase calculations thereforeshow the maximum vapor phase odorant concentration at high pressure. Ifa low potency odorant is used, like ethanol for example, it is necessaryfor it to be added to the fuel at very high loadings in order to bedetectable. A low potency odorant loaded at high concentration may notsatisfy the phase equilibrium requirements of the system. Thus, odorantloading is determined by both olfactory power and phase equilibrium,which may ultimately exclude a potential odorant as a possibility.

There is a minimum olfactory power and minimum vapor pressurerequirement of the odorant itself. As a basis for proper odorantloading, examine the natural gas case. The Gas Research Institute haspublished results on the variability of natural gas composition inmetropolitan areas. In this report, it is stated that mercaptans areadded to natural gas in varying amounts, ranging from 8 to 30 ppm, eventhough most mercaptans are easily detectable at sub-ppm levels.Furthermore, Japanese regulations state that odorants should be loadedin natural gas at 1000× the threshold detectability of an odorant. The“overloading” of odorant is due to variable human olfactory response andodorant condensation in pipes. In the survey performed by The GasResearch Institute, the average mercaptan loading in the U.S. isapproximately two orders of magnitude above the mercaptan threshold ofdetectability. Assuming the concentration ratio (odorant loading toodorant threshold) is approximately the same for the natural gas systemand the hydrogen system, loading requirements of different odorants withvarying olfactory thresholds may be calculated. This data may then befurther used to eliminate odorants with low olfactory thresholds, basedon odorant-hydrogen phase equilibrium. The loading factor from Table 3may be then used to calculate the maximum loading of odorants for theodorization of hydrogen, shown in Table 4. The saturated vapor phaseconcentration is then calculated for each of the odorants stored inhydrogen at 6000 psi. In Table 4, columns ‘A’ and ‘B’ show that ethanolcould perform as an effective odorant if loaded at 2884 ppm; however,ethanol would be in the vapor phase at concentrations no greater than190 ppm at 6000 psi. Thus, ethanol may be eliminated as an odorant as itis does not provide the olfactory power needed to meet saturated vaporphase concentrations. Generally, if values in column A are greater thanthe values in column B (A>B), then the odorant may be ruled out as apossibility. The situation described above may be generalized for allodorants given their vapor pressure and olfactory power.

TABLE 3 Odorant Loading Requirements in Natural Gas Minimum OdorantOlfactory Odorant Loading Required to Maximum Odorant Power ThresholdDetect H₂ at ⅕ LFL Loading in NG Loading Odorant (p. ol) (ppm)^(a)(ppm)^(b) (ppm)^(c) Factor^(d) ethyl mercaptan 8.97 0.00107 0.107 30 280^(a)Minimum detection concentration or odorant threshold (ppm) =10^((−odorant concentration)) × 10⁶ ^(b)Odorant threshold in hydrogen at⅕ the LFL (⅕ LFL = 1 vol. % H₂ in air). ^(c)Liss, W. E. et al.,Variability of Natural Gas Composition in Select Major MetropolitanAreas of the US, Gas Research Institute Report, Contract #5091-293-2132,1992. ^(d)Loading factor = maximum loading in NG/minimum loadingrequired for detectability.

TABLE 4 Sample Odorant Loading Requirements for Hydrogen Minimum Odorant‘A’ Required ‘B’ Saturated Olfactory Odorant Loading Required to OdorantLoading Odorant Loading Power Threshold Detect H₂ at ⅕ LFL^(b) inHydrogen [6000 psi H₂] Odorant (p. ol) (ppm)^(a) (ppm) (ppm)^(e)(ppm)^(f) ethanol 4.54 28.840 2884.03 807,529 190 pyridine 7.07 0.085118.5114 2,383 67 ethyl chloride 5.39 4.074 407.38 114,066 3256 ethylmercaptan 8.97 0.001072 0.1072 30 1705 ethyl selenol 10.74 0.0000180.0018 0.5 833 propyl amine 7.96 0.010965 1.0965 307 999 dimethyl amine7.09 0.081283 8.1283 2,276 4892 methyl amine 7.73 0.018621 1.8621 5218543 ^(e)Required loading in H₂ = loading factor (280) * minimum odorantloading required ^(f)Saturated odorant loading at 6000 psi = P_(A)^(sat)/P × 10⁶

Based on odorant saturation vapor pressures, a minimum effectiveolfactory power of an odorant may be determined. FIG. 4 shows therelationship between olfactory power and vapor pressure of an odorant.As the olfactory power decreases from 10 to 4, the detectableconcentration ranges from a few ppm required for detectability toseveral thousand ppm required for detectability. The detectable limitmay then be correlated to find the minimum vapor pressure required for aparticular odorant. From the olfactory power curve, of FIG. 4, anasymptote occurs at olfactory power approximately equal to 7. Therefore,this is the minimum olfactory power required for an acceptable odorant.When odorant properties are plotted on this graph and saturated vaporphase concentration at 6000 psi (y_(A)) is calculated from vaporpressure using Raoult's Law, any odorants that fall below the olfactorypower curve are excluded from further consideration. Any odorantsremaining above the olfactory power curve may remain as possible odorantcandidates. This mathematical correlation allows selection or rejectionof odorants based on olfactory power, vapor pressure, and saturatedvapor phase concentration.

Hydrogen utilization is a greater safety risk in confined areaconditions where there is potential for leaking hydrogen to collect inpockets. Because of its very small molecular size and weight, lowdensity, and high diffusivity, hydrogen readily disperses and rapidlymixes in air. This unique behavior presents additional concerns whenutilizing, transporting, and storing hydrogen in or near a confinedspace. However, relative to natural gas, or other flammable gases,hydrogen may not pose the same safety concerns in an open-airenvironment, where natural gas and propane are more dense fuels thattend to collect near the ground. During a leak event hydrogen dispersesinto the atmosphere very rapidly without the potential of collectinginto a flammable concentration.

Substance releases are classified into wide and limited aperturereleases. In a wide aperture case, a large hole develops, for example byover-pressurizing and explosion of a storage tank, and releases asubstantial amount of hydrogen in a very short amount of time. Inlimited aperture scenarios, material is released at a slow rate andupstream conditions are not immediately affected. Limited aperturereleases have multiple sources like holes, cracks in tanks and pipes,leaks in flanges, valves, and pumps, and severed or ruptured pipes.

During a leak event hydrogen may leak from a small crack, hole, orsevered pipe into a space with an initial velocity that quickly decayswith time until it reaches a value of zero. The leaking process reachessteady state in a short time. Leak velocity, pressure dissipation,temperature, convective eddies, and diffusion properties controldispersion throughout the space. To ensure the highest order of safetyfor the public, hydrogen dispersion and diffusion needs to beunderstood. Hydrogen's very low flammability and detonation limits,coupled with its dispersion characteristics, present a very dangerouscombination if hydrogen utilization systems are not properly designed.Hydrogen transport is an important element for the development andimprovement of hydrogen fueled systems. However, the transport behaviorof hydrogen relative to the behavior of an odorant is equally importantwhen selecting an effective hydrogen odorant.

The present invention includes a three-dimensional diffusion modeldeveloped to describe hydrogen and odorant transport during a leakevent. Hydrogen is the most diffusive molecule in air, so if the loadedodorant may prove to be effective during a leak event by enablingsensory detection in a diffusion limited environment, prior to hydrogenaccumulating to a flammable or explosive mixture, odorant detectionduring a convective dominant or a convective-diffusive mixing event iscertain.

The basis for the model is the general form of the advection/diffusionequation for the transport of substances through an environment:

Where (∂C/∂t) ultimately describes the concentration change of hydrogenwithin a confined space as time passes. (D_(i)∂C/∂i²) denotes thediffusive/dispersive phenomena as the concentration of hydrogen mixeswith air in the lateral and transverse directions both through molecularand mechanical effects. (V_(i)∂C/∂i) describes the advection of hydrogenas it moves and mixes with air. (λC) describes the mass generation ofhydrogen as it adsorbs to surfaces throughout the space.

A three-dimensional numerical diffusive model was developed to simulatehydrogen leaking from a point source within a confined space. Thesimulation also takes into account no mass generation.

The resulting general equation is as follows:

$\frac{\partial C}{\partial t} = \left\lbrack {{D_{x}\frac{\partial^{2}C}{\partial x^{2}}} + {D_{y}\frac{\partial^{2}C}{\partial y^{2}}} + {D_{z}\frac{\partial^{2}C}{\partial z^{2}}}} \right\rbrack$The Forward-Time-Central-Space explicit scheme utilized is as follows:

$\frac{C_{i}^{n + 1} - C_{i}^{n}}{\Delta\; t} = {\alpha\left\lbrack \frac{C_{i + 1}^{n} - {2C_{i}^{n}} + C_{i - 1}^{n}}{\left( {\Delta\; x} \right)^{2}} \right\rbrack}$

The code was written to enable conditional changes to be made to thesize of the confined space, the diffusion coefficient, the position ofthe leak source within the space, and the position by which an ignitionsource may be present. Therefore, the model conservatively describesmultiple scenarios chosen by the user. Since it is necessary todetermine the time it would take for a fire hazard to accumulate, themodel has been designed to discretely focus on describing aconcentration change reaching the lower flammability limit of 4%hydrogen to air. A 4% hydrogen concentration is critical as it is theminimum characteristic H₂/Air ratio by which flame propagation issupported. Two point sources where considered as shown in FIG. 5: (1) asource at the center of the confined space; and (2) a source in a cornerof the confined space.

These two points were selected for comparisons of (1) where there is noreflection of hydrogen, and (2) where hydrogen reflects off the walls inthe corner of the room. Three observation points where considered torepresent ignition sources equidistant away from each source point. Theresults obtained from the simulation may be used for comparison for eachsource scenario. The dimensions of each source, and observation pointsare provided below:

Coordinate System:

Point Sources: Observation Points: Location of 1: Location of A: 2.8 m ×0.6 m × 3.3 m 2.5 m × 0.3 m × 3.0 m Location of B: 3.1 m × 0.9 m × 3.6 mLocation of 2: Location of C: 3.4 m × 1.2 m × 3.9 m 4.7 m × 0.3 m × 5.7m Location of D: 4.4 m × 0.6 m × 5.4 m Location of E: 4.1 m × 0.9 m ×5.1 m Location of F: 3.8 m × 1.2 m × 4.8 m

Referring to FIG. 5, the leak sources are denoted with * symbol and theobservation points are denoted by dots •. The confined space represents,for example, a 1-car garage with the dimensions of 5.0 m×3.0 m×6.0 m.This scenario was selected as an area wherein a hydrogen source forfueling both home and vehicle fuel cell systems may be found. However,more importantly a garage is an example of a confined space for thebasis of the analysis.

The first leak source was selected to be in the center of the roomapproximately 1 ft above the floor. The second was placed approximately1 ft in the x, y, and z directions from the back, right corner of thegarage. Both sources were chosen to compare with observation points atequal distances away. All observation points are equidistant from thesource and each other by approximately 1 ft (0.30 m) in the x, y, and zplanes according to Groups (Group 1, center of garage; Group 2, cornerof garage). The model provides the capabilities to choose where the leaksource is located as well as where possible ignition sources mayaccumulate. The model also provides the time at which a 4% concentrationof hydrogen/air reaches the ignition point, therefore, further providingan understanding of critical time limits by which the odorant should bedetected relative to any position in a confined space. As displayed inFIG. 6 and Table 5, more time is required for the 4% hydrogenconcentration to develop as the distance from each source increases.

TABLE 5 Hydrogen Flammability Limit Concentration Data SummaryObservation Time to Reach H₂ Flammability (4% H₂/Air) Point (s) (min)(hrs) A 1566 26.1 0.44 B 61288 1021.5 17.02 C 99520 1658.7 27.64 D 196432.7 0.55 E 15683 261.4 4.36 F 39996 666.6 11.11

As explained above, source example (1) and (2) are compared to reviewthe time difference for hydrogen concentration developments to reach thelower flammability limit in the center of the room and in a corner,respectively. In FIGS. 7, 8, and 9, the trend lines are labeled byobservation point. Observation point A, B, and C correspond to sourcescenario (1) for flammability limit concentration development times inthe center of the room. Observation point D, E, and F correspond tosource scenario (2) for flammability limit concentration developmenttimes near the corner of the room FIG. 7 shows a comparison of hydrogenconcentration increasing with time for observation points A and D. Theconcentration profiles for both observation points are almost identical.It must be noted that these times are conservative (transport bymolecular diffusion in a stagnant air environment) for comparing odorantdetection. Even though it takes twenty minutes to reach a flammableconcentration, the hydrogen concentration front reached the observationpoints in seconds. FIG. 8 shows a comparison of hydrogen concentrationincreasing with time for observation points B and E. It takes less timefor the concentration to reach 4% at E as compared to B, and as a resultthe concentration profile E is much steeper than profile B. By thistime, the wall perpendicular to the x and z-axes become source pointsdue to hydrogen reflection. FIG. 9 shows a similar comparison of howhydrogen concentration increases with time for observation points C andF. FIG. 9 further demonstrates that the flammable concentration timesfor a leak scenario in a more open space, as opposed to a scenario wherereflection occurs, begins to diminish as distance between theobservation point and the source increases.

Odorant detectability was analyzed and compared to review how thepreliminary odorant selections behave during transport relative tohydrogen. The times by which the odorants reach detectability limitswere simulated using the existing three-dimensional transport model.

It is possible to determine the detectability of the odorant based onthe loading of the odorant within the tank. As an example, a tank may beloaded with 0.1% odorant relative to 99.9% hydrogen. For modelingpurposes, 0.1% (1000 ppm) of odorant represents 100% of the sourcerelease during a leak. The detectability of ethylselenol (a preferredodorant) is 1.8E-5 ppm based on its olfactory power. Therefore,fractional detectability (Y) for ethylselenol is calculated as follows:

$\frac{100}{X} \propto \frac{Y}{{1.8E} - {5\mspace{14mu}{ppm}}}$Olfactory power is the potency rating given to an odorous compound inair. X is the loading concentration in ppm, and Y is the fractionaldetectability. Therefore, if X=1000 ppm as described above, fractionaldetectability (Y) may be solved for where Y is equal to 1.8E-6 ppm.Fractional detectability is a scaling ratio and is unitless. Therefore,fractional detectability may be used in the model to calculate odorantdetectability time. This comparison was made to estimate the times bywhich the odorant becomes detectable at the observation points, shown inFIGS. 3 and 4, and to compare the times by which the odorantconcentration profile reaches observation points relative to hydrogen.Diffusion coefficients were predicted for a binary gas system at lowpressure by the Boltzmann equation utilizing collision integrals (σ) andLennard-Jones Potentials (ε). The collision integrals and theLennard-Jones Potentials were determined through viscosity data. Table 6shows the estimated times by which the odorant detectabilityconcentration profile reaches each of the observation points relative tohydrogen.

TABLE 6 Odorant Detectability Data Summary Loading Observation (A) Det.Time Observation (B) Det. Time Observation (C) Det. Time Observation (D)Det. Time Conc. (ppm) Frac. Det (s) (min) (hrs) (s) (min) (hrs) (s)(min) (hrs) (s) (min) (hrs) EthylSelenol Detectability Data Summary 10001.80E−06 117.50 1.96 0.03 1739.00 28.98 0.48 3833.00 63.88 1.06 252.004.20 0.07 100 1.80E−05 168.00 2.80 0.05 2189.00 36.48 0.61 4761.00 79.351.32 343.00 5.72 0.10 30 6.00E−05 205.00 3.42 0.06 2505.00 41.75 0.705415.00 90.25 1.50 408.00 6.80 0.11 20 9.00E−05 220.00 3.67 0.06 2628.0043.80 0.73 5671.00 94.52 1.58 433.00 7.22 0.12 10 1.80E−04 247.00 4.120.07 2863.00 47.72 0.80 6164.00 102.73 1.71 480.00 8.00 0.13 5 3.60E−04280.00 4.67 0.08 3137.00 52.28 0.87 6740.00 112.33 1.87 535.00 8.92 0.152 9.00E−04 331.00 5.52 0.09 3574.00 59.57 0.99 7666.00 127.77 2.13622.00 10.37 0.17 0.5 3.60E−03 435.00 7.25 0.12 4478.00 74.63 1.249603.00 160.05 2.67 797.00 13.28 0.22 EthylMercaptan Detectability DataSummary 1000 1.10E−04 239.00 3.98 0.07 2834.00 47.23 0.79 6112.00 101.871.70 469.00 7.82 0.13 100 1.10E−03 362.00 6.03 0.10 3878.00 64.63 1.088316.00 138.60 2.31 678.00 11.30 0.19 30 3.67E−03 459.00 7.65 0.134730.00 78.83 1.31 10143.00 169.05 2.82 842.00 14.03 0.23 20 5.50E−03500.00 8.33 0.14 5093.00 84.88 1.41 10932.00 182.20 3.04 911.00 15.180.25 10 1.10E−02 582.00 9.70 0.16 5847.00 97.45 1.62 12583.00 209.723.50 1050.00 17.50 0.29 5 2.20E−02 686.00 11.43 0.19 6828.00 113.80 1.9014764.00 246.07 4.10 1225.00 20.42 0.34 2 5.50E−02 871.00 14.52 0.248685.00 144.75 2.41 18996.00 316.60 5.28 1543.00 25.72 0.43 0.5 2.20E−011355.00 22.58 0.38 14155.00 235.92 3.93 32288.00 538.13 8.97 2385.0039.75 0.66 MethylAmine Detectability Data Summary 1000 1.86E−03 294.004.90 0.08 3080.00 51.33 0.86 6601.00 110.02 1.83 544.00 9.07 0.15 1001.86E−02 482.00 8.03 0.13 4798.00 79.97 1.33 10359.00 172.65 2.88 862.0014.37 0.24 30 6.20E−02 659.00 10.98 0.18 6575.00 109.58 1.83 14412.00240.20 4.00 1166.00 19.43 0.32 20 9.3E−2  742.00 12.37 0.21 7470.00124.50 2.08 16518.00 275.30 4.59 1310.00 21.83 0.36 10 1.86E−01 932.0015.53 0.26 9638.00 160.63 2.68 21813.00 363.55 6.06 1641.00 27.35 0.46 53.72E−01 1222.00 20.37 0.34 13314.00 221.90 3.70 31409.00 523.48 8.722144.00 35.73 0.60 2 9.30E−01 1944.00 32.40 0.54 24836.00 413.93 6.9065536.00 1092.27 18.20 3339.00 55.65 0.93 0.5 3.72E+00 6839.00 113.981.90 256769.00 4279.48 71.32 442098.00 7368.30 122.81 8924.00 148.732.48 Loading Observation (E) Det. Time Observation (F) Det. Time Conc.(ppm) Frac. Det (s) (min) (hrs) (s) (min) (hrs) EthylSelenolDetectability Data Summary 1000 1.80E−06 2181.00 36.35 0.61 4487.0074.78 1.25 100 1.80E−05 2731.00 45.52 0.76 5555.00 92.58 1.54 306.00E−05 3115.00 51.92 0.87 6304.00 105.07 1.75 20 9.00E−05 3265.0054.42 0.91 6596.00 109.93 1.83 10 1.80E−04 3552.00 59.20 0.99 7156.00119.27 1.99 5 3.60E−04 3885.00 64.75 1.08 7804.00 130.07 2.17 2 9.00E−044415.00 73.58 1.23 8837.00 147.28 2.45 0.5 3.60E−03 5498.00 91.63 1.5310941.00 182.35 3.04 EthylMercaptan Detectability Data Summary 10001.10E−04 3520.00 58.67 0.98 7105.00 118.42 1.97 100 1.10E−03 4787.0079.78 1.33 9574.00 159.57 2.66 30 3.67E−03 5806.00 96.77 1.61 11554.00192.57 3.21 20 5.50E−03 6236.00 103.93 1.73 12388.00 206.47 3.44 101.10E−02 7115.00 118.58 1.98 14091.00 234.85 3.91 5 2.20E−02 8231.00137.18 2.29 16254.00 270.90 4.52 2 5.50E−02 10249.00 170.82 2.8520185.00 336.42 5.61 0.5 2.20E−01 15509.00 258.48 4.31 30683.00 511.388.52 MethylAmine Detectability Data Summary 1000 1.86E−03 3793.00 63.221.05 7570.00 126.17 2.10 100 1.86E−02 5799.00 96.65 1.61 11458.00 190.973.18 30 6.20E−02 7729.00 128.82 2.15 15219.00 253.65 4.23 20 9.3E−2 8642.00 144.03 2.40 17016.00 283.60 4.73 10 1.86E−01 10704.00 178.402.97 21143.00 352.38 5.87 5 3.72E−01 13788.00 229.80 3.83 27504.00458.40 7.64 2 9.30E−01 21205.00 353.42 5.89 43831.00 730.52 12.18 0.53.72E+00 68859.00 1147.65 19.13 173127.00 2885.45 48.09

The present invention includes an odorant detectability analysis thatdetermines the minimum limits for odorant loading based on dispersion.Minimum odorant loading limits were also established with respect tovapor phase and olfactory relationships. Both criteria should befollowed. Currently, codes and standards are under the developmentalstages to deliver minimum safety requirements by which manufacturers,distributors, and hydrogen systems will need to comply with. Oneregulation will require the capability to detect a hydrogen plume at aminimum of 20% of its lower flammability limit (LFL). This is calculatedto be 0.82% H₂/Air.

The minimum loading concentration of an odorant sufficient to bedetected when hydrogen reaches 20% of its lower flammability limit isobtained by plotting odorant detection time (y-axis) versus odorantloading concentration (x-axis). The value where the odorantdetectability curve (corresponding to loading concentration) interceptsthe time by which hydrogen reaches 20% of its lower flammability limitis the minimum loading concentration sufficient for detection. Thesevalues are summarized in Tables 7, 8, and 9, for three sample odorants.

TABLE 7 Ethylselenol 20% LFL and Detectability Intercept Summary OdorantLoading Required to Meet 20% LFL H₂/Air Observation Intercept SummaryPoint (ppm) A 0.95 B 0.32 C 0.22 D 1.7 E 2.7 F 2.5

TABLE 8 Ethylmercaptan 20% LFL and Detectability Intercept SummaryOdorant Loading Required to Meet 20% LFL H₂/Air Observation InterceptSummary Point (ppm) A 98 B 38 C 10.4 D 110 E 260 F 200

TABLE 9 Methylamine 20% LFL and Detectability Intercept Summary OdorantLoading Required to Meet 20% LFL H₂/Air Observation Intercept SummaryPoint (ppm) A 325 B 130 C 60 D 325 E 575 F 500

In some applications, in may be necessary to remove the odorant in thehydrogen energy system, since, for example, the fuel cell catalyst andstorage technologies (metal hydrides, carbon nanotubes, and glassmicrospheres) generally require ultra-pure hydrogen. In addition, theodorant should be captured before the exhaust of the fuel cell system inorder to avoid a false leak warning. Various technologies may beemployed to remove the odorant from hydrogen. An important feature ofthe present invention provides for odorant removal prior to systemcomponents that are sensitive to impurities in the hydrogen stream. Morespecifically, an odorant may be removed through a fixed bed adsorptioncolumn just prior to a solid storage unit (metal hydrides, carbonnanotubes, or glass microspheres), or in the case of compressed hydrogenstorage, the odorant may be removed just prior to the fuel cell system.

Generally, a system design includes hydrogen production, a source ofsupplied hydrogen (compressed gas), an on-site or on-board hydrogenstorage unit, odorant adsorbers, and a fuel cell stack. As seen in FIGS.10 and 11, the process includes the removal of odorant throughadsorbers. In FIG. 10, the odorant adsorbers are placed just prior tothe solid storage unit, whereas in FIG. 11, the adsorbers are placedjust prior to the fuel cell stack. In both cases fixed-bed adsorptionmay be employed to remove the odorant from hydrogen. Although simplephysical adsorption may be effective for removal of an odorant, chemicaladsorbents may offer superior capacities, and thus may offer a longerlifetime. It is expected than any adsorbent column would need to bereplaced periodically.

The Rosen Model, developed by J. B. Rosen in 1952, provides ananalytical solution to adsorption kinetics in a linear fixed bedadsorption column. This model is suitable for predicting the adsorptionkinetics for removal of trace organic impurities from a gas stream, andthe rate of adsorption is governed by two effects: (1) mass transfer ofthe adsorbate from the bulk gas phase to the surface of sphericalparticles, and (2) diffusion into the pores of spherical particles. Anempirical parameter for equilibrium adsorption capacity is required,based upon Henry's Law. According to Rosen, all other processesaffecting the kinetics of the system are very rapid compared todiffusion of the adsorbate either to the surface of the adsorbentparticle or within the adsorbent particle itself. Thus, effects (1) and(2) are rate limiting. FIG. 12 shows a simple schematic of the systemunder consideration. The following variables are considered in Rosen'scalculations:

Adjustable Parameters:

K—equilibrium adsorption constant, slope of linear isotherm (Henry'sLaw)

b—radius of spherical particles, cm

D—effective coefficient of diffusion in spherical particles, cm²/s

k—effective mass transfer coefficient, m/s

Other Nomenclature:

c—concentration of solute in fluid, meq./cc. fluid

c_(o)—constant value of c at column entrance, meq./cc. fluid

q—concentration of solute in solid particles, meq./cc. solid

m—void volume/unit volume solid particles

t—time from start of process, s.

u—c/c_(o)

v—linear flow velocity, cm/s.

x—3DKz/mvb², bed length parameter, dimensionless

y—2D(t−z/v)/b² contact time parameter, dimensionless

z—distance from column inlet, cm

v—DK/kb, film resistance parameter, dimensionless

Rosen assumes that (1) particles are spherical with uniform radius; (2)constant linear flow velocity—any variations in concentration or flowvelocity over a given cross section may be neglected and longitudinaldiffusion is assumed to be negligible; (3) mass transfer coefficient andpore diffusion coefficient are independent of position and ofconcentration; and (4) system has a linear isotherm. Under equilibriumconditions, the concentration of adsorbed material at the solid surfaceis given by Henry's Law, q=Kc. For the model to be successful andaccording to this condition, the feed stream must have a lowconcentration (partial pressure) of adsorbate.

With these assumptions, the solution is linear and may be solved toobtain the governing equation for combined surface film and internalsolid diffusion. Rosen's equation:

$u = {0.5\left\lbrack {1 + {{erf}\left( \frac{\frac{3y}{2x} - 1}{2\sqrt{\frac{1 + {5v}}{5x}}} \right)}} \right\rbrack}$

The Rosen equation was solved and plotted over the appropriate timedomain. A plot of u(c/c_(o)) as a function of time yields a breakthroughcurve for the set of parameters chosen. The effect of each adjustableparameter must be determined to optimize adsorption for a givenadsorption material, bed size, odorant and flow rate. Mass transfercoefficients for adsorbate traveling from the bulk gas stream to theadsorbent surface may be approximated to be 15 m/s for most smallorganic molecules. Effective diffusivity coefficients may also beapproximated based on typical values for molecular Knudsen diffusion ofsmall organic molecules (˜10⁻⁸ m²/s) in pores. With a relatively largemass transfer/large diffusivity terms, the standard breakthrough curvetends toward a step increase in outlet concentration of odorant atbreakthrough. When these terms are relatively very small, thebreakthrough curve tends toward a sloping “S” shape. The effect of theseparameters on breakthrough time is illustrated in FIGS. 13 and 14. Themain parameter affecting breakthrough times is the equilibriumadsorption capacity of the adsorbent itself. As an example, if material“A” adsorbs more odorant than material “B” under equilibrium conditions,then material “A” also shows much longer breakthrough times. Equilibriumadsorption capacities are difficult to predict and as a result must beobtained from experiment. The effect of increasing Henry's Law constanton breakthrough times is illustrated in FIG. 15. This analysisquantitatively determines the effects of different parameters governingbreakthrough times according to the Rosen model.

The Rosen model provides a solution to a multi-parameter adsorptionproblem and requires empirical data to obtain breakthrough curves for aparticular odorant/adsorbent system. Empirical Henry's law constants arerare for exotic odorous compounds, so a conservative scenario shows thatfixed bed adsorption is feasible for removing the odorant. H₂S, whichhas a lower molecular weight and a high vapor pressure. As a result, H₂Sis less condensable on the adsorbent as compared to odorants of thepresent invention. Higher molecular weight odorants are expected to havemuch higher Henry's constants. Therefore, this scenario is conservativein determining adsorbent bed size and breakthrough times.

TABLE 10 Empirically determined Henry's law constants for variousadsorbent/adsorbate systems. BET Equilibrium Commercial Pore SurfaceAdsorption K_(H) Adsorbent Volume Area Capacity (dimen- Type Adsorbate(cm³/g) (m²/g) (mg/g) sionless) BAX-1500^(a) H₂S 1.214 1400 295 3386WVA-1100^(a) H₂S 1.180 1110 230 2640 ^(a)manufactured by Westvaco (woodbased, H₃PO₄ activated)

The characteristics of the solid adsorbent (surface area, porestructure, and surface chemistry) are important parameters determiningequilibrium adsorption capacities of these materials. Higher surfacearea materials are capable of adsorbing substantially more odorant ascompared to materials with lower surface area.

For the lower surface area adsorbent (WVA-1100), adsorption capacity isless and therefore, breakthrough may occur at shorter times.Breakthrough times significantly increase for higher surface areaadsorbents. The results for the different activated carbons produced byWestvaco are shown in FIG. 16 for adsorption of H₂S. The K_(H) valuesreflect the initial concentration of odorant in the gas phase, as wellas equilibrium adsorption capacities of the adsorbents. H₂S was loadedin hydrogen at 3000 ppm concentration and passed through the bed at aflow rate of 400 L/min, and the adsorbent bed is 1 m in length and 30 cmin diameter, holding 64 kg of activated carbon. Both high surface areacarbons give significant breakthrough times. The breakthrough times of6650 s (110.62 min) and 9250 s (154.3 min) correspond to 7 and 10 tankrefills respectively. Given the large flow rates through the bed, thesevalues seem reasonable for an H₂S concentration of 3000 ppm. After theadsorbent bed is saturated, it needs to be replaced.

For an odorant with a lower vapor pressure compared to H₂S, longerbreakthrough times may be expected. In addition, larger molecules mayhave a greater surface interaction with the carbon surface and may beretained to a greater degree compared to H₂S.

Reviewing the selection criteria, as discussed herein, a suitableodorant may be selected. The odorant should have a strong smell, unlikethat of mercaptans (thiols) and sulfides that are used to odorizenatural gas. The hydrogen odorant should have a unique and unpleasantsmell, detectable at less that 1 ppm. Odorant olfactory power and vaporpressure are related to the amount of odorant in the vapor phase at 6000psi. The odorant should pass the olfactory power-vapor pressure test asdescribe in detail above. The odorant should possess either sufficientolfactory power or diffusivity in order to be detectable before hydrogenreaches its flammability limits. The odorant should not be hazardous orcause harmful health effects in humans at the levels loaded in hydrogen.A selection matrix may then be created from the above-mentionedcriteria. The selection matrix either keeps the odorant as a candidateor rejects it as a possible odorant. The selection matrix is given inFIG. 17. Based on these criteria, preferred odorants are listed in Table11 below.

TABLE 11 Short-list of Odorants for the Hydrogen Economy MW # OdorantChemical Formula (g/mol) Selenium Compounds 1 methyl selenol CH3SeH95.01 2 ethyl selenol C2H5SeH 109.04 3 dimethyl selenide CH3SeCH3 109.044 isoproyl selenol C3H7SeH 123.07 5 propyl selenol C3H7SeH 123.07 6ethylmethyl selenide C2H5SeCH3 123.07 7 isopropylmethyl selenideC3H7SeCH3 137.10 8 tertbutyl selenol C4H9SeH 137.10 9 diethyl selenideC2H5SeC2H5 137.10 Nitrogen Compounds 10 methyl amine CH3NH2 31.06 11ethyl amine C2H5NH2 45.09 12 dimethyl amine (CH3)2NH 45.09 13 propylamine C3H7NH2 59.12 14 ethylmethyl amine C2H5NH(CH3) 59.12 13 trimethylamine (CH3)3N 59.12 Oxygen Compounds 14 2,3-butanedione C4H6O2 86.09 15ethyl acrylate C5H8O2 100.12

All documents, patents, journal articles and other materials cited inthe present application are hereby incorporated by reference.

Although the present invention has been fully described in conjunctionwith the preferred embodiment thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. A fuel cell containing a hydrogen composition comprising: hydrogen;and an odorant, said odorant having a vapor pressure greater than 0.5psi and having a smell detectable at less than 1 ppm by a human nose,wherein said odorant is a selenium compound.
 2. A fuel cell containing ahydrogen composition comprising: hydrogen; and an odorant, said odoranthaving a vapor pressure greater than 0.5 psi and having a smelldetectable at less than 1 ppm by a human nose, wherein said odorant is aselenium compound and wherein said fuel cell is a vehicle fuel cell. 3.The fuel cell of claim 1, wherein said selenium compound isethylselenol.
 4. The fuel cell of claim 1, wherein said seleniumcompound is dimethyl selenide.
 5. The fuel cell of claim 1, wherein saidgaseous composition consists essentially of hydrogen gas and saidodorant.
 6. The fuel cell of claim 1, wherein said odorant comprises0.01 to 1000 ppm of said composition.
 7. The fuel cell of claim 1,wherein said odorant comprises 0.1 to 40 ppm of said composition.
 8. Thefuel cell of claim 1, wherein said odorant is not harmful to humans. 9.The fuel cell of claim 1, wherein said odorant has a minimum olfactorypower of 7.0, a minimum vapor pressure of 0.5 psi at standardtemperature and pressure, a minimum diffusivity of 0.01147 cm.sup.2/s,and a maximum molecular weight of 200 g/mol.
 10. The fuel cell of claim1, wherein said selenium compound is methylselenol.
 11. The fuel cell ofclaim 1, wherein said selenium compound is isopropylselenol.
 12. Thefuel cell of claim 1, wherein said selenium compound is propylselenol.13. The fuel cell of claim 1, wherein said selenium compound isethylmethyl selenide.
 14. The fuel cell of claim 1, wherein saidselenium compound is isopropylmethyl selenide.
 15. The fuel cell ofclaim 1, wherein said selenium compound is tertbutylselenol.
 16. Thefuel cell of claim 1, wherein said selenium compound is diethylselenide.
 17. The fuel cell of claim 1, wherein said odorant is in avapor phase at a pressure greater than ambient pressure.